This invention relates to imaging apparatus in which an image of a scene is formed from nonvisible radiation from the scene, and in which a radiation detector is scanned across the image. A visible version of the scene is reconstructed by a corresponding scan of a light source modulated by the detector output. More particularly, the invention relates to thermal imaging apparatus in which the nonvisible radiation is the natural thermal infrared radiation emitted by objects in the scene by reason of their temperatures and emissivities.
A thermal imaging apparatus is disclosed in U.S. Pat. No. 3,626,091. The apparatus comprises a linear array of infrared detectors and a corresponding linear array of light sources. Each light source is modulated by a corresponding detector output. The detectors and light sources are synchronously scanned in a plurality of bands of lines across the scene by a rotating mirror prism having planar mirrors set at various angles around the axis of rotation. The mirror prism has twice as many mirror faces as there are bands scanned in the scene. Scanning of the detectors is carried out by one set of mirrors while reconstruction (scanning of the light sources) is carried out by a second set of mirrors. British Pat. No. 2,087,189A describes thermal imaging apparatus of this type in which a mirror prism is used, and in which all but one of the planar mirrors take part in both scanning and reconstruction.
In thermal imaging apparatus of this type, the detectors, commonly photoconductive indium antimonide or cadmium mercury telluride, are usually cooled and usually have a cold shield to limit their field of view. Liquid nitrogen, solid carbon dioxide or thermoelectric cooling means may be employed. In consequence, the radiation emitted by the detectors themselves, by the substrate upon which they are mounted and by the cold shield will be generally less than that emitted by adjacent uncooled parts of the inside of the apparatus.
An objective lens is used to form an infrared image of the scene upon the detectors. Typically, this objective will be stationary relative to the detectors, and the planar scanning mirrors are beyond the objective.
An afocal telescope may be placed beyond the scanning mirrors to provide telefoto or wide-angle viewing. Typically an afocal refracting telescope may be used comprising germanium and silicon elements, as described and claimed in British Pat. No. 1,530,066. The optical elements of this telescope frequently comprise surfaces concave toward the detectors. Germanium and silicon are of relatively high refractive index (3.4. and 4.0 respectively), and the amplitude of thermal radiation reflected by such surfaces is not negligible, even when these surfaces have been coated to minimize such reflection.
Consequently, during a part of the scan, usually the center, the detectors receive radiation emitted by themselves, the substrate, and the cold shield, which radiation has been reflected by at least one surface of the objective lens or, via the scanning mirror, by at least one surface of the telescope elements. This reflected radiation is reduced in amount compared to that which the detectors receive during the remainder of the scan from adjacent uncooled parts of the inside of the apparatus. In consequence, the general level of the detectors' outputs falls during the center part of the scan, darkening the general level of picture detail and creating the appearance of a dark patch in the center of the picture. To those skilled in the art, this is known as the "narcissus" effect, referring to the fact that it is due to the cooled detectors "seeing" themselves by reflection.
Also, if the thermal imager is being used to view a generally cold scene, a considerable drop in detector output occurs as the scanning mirror passes from the inside of the equipment onto the scene.
Relatively large changes in brightness can occur across naturally occurring thermal scenes, making it difficult to display in one picture details of dark parts and high lights simultaneously. Flattening the general level of brightness across the visible scene may be of advantage in some applications of thermal imaging.